In Jan. 1998 the European Telecommunications Standards Institute (ETSI) chose the basic technology for the Universal Mobile Telecommunications System (UMTS) (see ETSI, “The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candidate Submission,” June 1998). The main proposed radio interface was the Wideband Code Division Multiple Access (WCDMA) protocol, the features of which offer the opportunity for fully meeting the requirements of third generation (3G) mobile telephony. Because of the high data transmission rate and the increasingly stringent quality of service (QoS) requirements in 3G, the development of new planning strategies were required. These included the power control system, probably that of greatest study, used to implement the outer loop of the system.
The following describes this power control system in general terms since the operation of the outer loop, for which this invention proposes a method, is the result of other components in the system.
The power control system in cellular networks based on WCMDA is required because the technology is limited by interference since all users share the same frequency spectrum and their codes are not completely orthogonal (see Holma & Toskala: “WCDMA by UMTS, Radio Access for Third Generation Mobile Communications,” John Wiley & Sons).
The final purpose of the power control system in WCDMA is to obtain the quality of service required in a specific connection, the downlink from the base station to the mobile or terminal, or the uplink from the mobile to the base station, with a minimum level of transmitted power (the precise aspect on which this invention is centred).
The main objectives of the power control system in WCDMA networks are:
Cancellation of the near/far effect: if all the mobile stations were to transmit at the same power without considering the distance or the fading to the base station, the mobiles closest to it would significantly interfere with the terminals further away.
Protection against severe fading.
Minimising the interference in the network with the consequent improvement in capacity.
Longer battery life in the mobile stations.
There are three procedures for implementing a power control system for WCMDA:
By open loop: During the random access process at the start of a connection, the base/mobile station estimates the power loss in the uplink/downlink and adjusts its and transmission power as a function of this.
By closed or inner loop: also called rapid power control (1500 Hz) which consists of the following three steps:
1) The relevant receiver (the base station or the mobile unit) compares the value of the desired signal to interference ratio received (SIRrec) to the desired signal to interference ratio target (SIRtarget) which depends on the quality of service required for this specific connection and which is set by the outer loop procedure, explained below.
2) The same receiver sends power control bits indicating that the power must be increased (if SIRrec<SIRtarget) or reduced (if SIRrec>SIRtarget) by a certain value (normally 1 dB).
3) The transmitter (base or mobile station) increases or decreases its power by the amount set previously.
By outer loop (OLPC, Outer Loop Power Control): this is much slower than the closed loop (10-100 Hz) and sets the desired signal to interference ratio target (SIRtarget) to maintain a pre-set quality objective. One criterion or measurement of the quality of a connection is the frame error rate (FER) or its equivalent, the block error rate (BLER), which is a function of the required signal to interference ratio (SIR). Given that the inner loop helps to maintain the desired signal to interference ratio received (SIRrec) close to the target (SIRtarget), the block error rate (BLER) is, in the end, determined by this target value. Thus, to achieve a quality of service in a specific fading environment, the target (SIRtarget) must be adjusted to the value that is suitable for this environment.
Sometimes, either because the channel conditions suddenly worsen, so that the receiver does not receive the power control bits sent by the transmitter, or because the transmitter has reached the maximum power available for this connection, the desired signal to interference ratio received (SIRrec) may always be lower than the desired signal to interference ratio target (SIRtarget).
The result of this situation is that the received frame error rate (FERrec) is greater than the target frame error rate (FERtarget), that is, that the quality of the connection is degraded. However, this degradation may not be large enough to cut the communication, so that it is maintained albeit with a quality that is less than that desired.
If this occurs—communication continues but at degraded quality—the so-called outer loop wind-up condition or mode may occur: the outer loop power control (OLPC) method will increase the desired signal to interference ratio target (SIRtarget), to try to reach the target quality criterion, that is, the target frame error rate (FERtarget), but the desired signal to interference ratio received (SIRrec) will not be able to follow the desired signal to interference ratio target (SIRtarget) for the reasons described above (worsening of the channel's conditions or saturation of the transmitter).
In this situation, the outer loop power control (OLPC) method will continue to increase the desired signal to interference ratio target (SIRtarget) indefinitely to a level much higher than before the sudden worsening of the propagation conditions or the power limitation.
When the conditions subsequently improve or the power limitation disappears, the desired signal to interference ratio received (SIRrec) will finally reach the desired signal to interference ratio target (SIRtarget), which will then have a much higher value than that for the target frame error rate (FERtarget). As a result, the received frame error rate (FERrec) being achieved at this point will be much less than necessary and, therefore, will be increasing the interference in the channel, reducing capacity and degrading the quality in other connections.
This undesirable situation will be maintained until the outer loop power control (OLPC) method manages to lower the desired signal to interference ratio target (SIRtarget) to a suitable value, that is, to that at which the target frame error rate (FERtarget) is achieved.
This process of lowering the desired signal to interference ratio target (SIRtarget) after the end of the condition described above, that is, after the wind-up, is called the outer loop unwinding condition or mode and this invention proposes a method for specifically this mode.
The problem is that due to the properties of the outer loop power control (OLPC) algorithm used normally (see Holma H, Toskala A, “WCDMA for UMTS,” Wiley, 2002), the process of lowering the desired signal to interference ratio target (SIRtarget) is very slow. This slow convergence is because the down step size used by the algorithm is, measured in dB, of the order of the target frame error rate (FERtarget) (typical values are 10−2 for the voice service and 10−3 for the video calls service), that is, very small, which means that dozens of seconds are needed for each dB decrease.
It is interesting to note that there is a disparity between the down step size and the up step size of the desired signal to interference ratio target (SIRtarget) fixed by the outer loop power control (OLPC) method. In fact, the up step size is much greater than the down step size: while, as mentioned, the latter is of the order of the target frame error rate (FERtarget) in dB, the up step size is approximately 1 dB. This fact is relevant because it implies that the outer loop power control (OLPC) method can react quickly to situations requiring an increase in the desired signal to interference ratio target (SIRtarget) and this is taken into account in this invention.
Because of the above, various answers have been devised to prevent the outer loop power control phenomenon (see US patent application 2003148769). This document proposes the following method for detecting the wind-up: a wind-up situation is declared when the difference between the desired signal to interference ratio target (SIRtarget) and the desired signal to interference ratio received (SIRrec) exceeds a specific margin or threshold. When the wind-up mode is detected, in US 2003148769, different mechanisms are set up to limit the value of the desired signal to interference ratio target (SIRtarget) while the wind-up situation lasts. Finally, also in this patent application, criteria are set for detecting the unwinding process, described above, that is, the end of the wind-up, and that starts when the desired signal to interference ratio received (SIRrec) is able to reach the desired signal to interference ratio target (SIRtarget).
However, in the power control described in US 2003148769, no criterion is set for the unwinding process itself, that is, only a possible form for detecting it is defined but no specific operation of the outer loop in this mode is described; it is assumed to match the normal operating mode of this loop or that of a very slow decrease with the resulting increase in interference in the channel, reduction of capacity and deterioration of the quality of other connections, as explained above.
It should be noted that there is a method to prevent the outer loop wind-up from attenuating the later unwinding process, as achieved in the quoted example of US 2003148769, but it does not prevent it.